The ability to adapt to a stress is a fundamental physiologic principle. By intentionally imposing a particular stress on animals, tissues, or cells, and studying the endogenous protective responses so induced, investigators can uncover new strategies for therapeutic intervention. Recently, this approach has been leveraged in the field of ischemia-reperfusion, where a robust "tolerance" to ischemic injury that is dependent on changes in gene expression can be triggered by pre-exposure to brief, noninjurious ischemia or hypoxia. [unreadable] [unreadable] To date, such "preconditioning" treatments are singular in nature, and the resulting duration of tolerance is short lasting. We hypothesized that repetitive preconditioning treatments would confer a much longer lasting period of ischemic tolerance in the tissue. Indeed, results of our recent studies in mice using repetitive presentations of mild systemic hypoxia or multiple injections of the hypoxia-mimetic deferroxamine to precondition the retina support this hypothesis. By both morphologic and functional criteria, we can document a neuroprotective phenotype that lasts for months instead of days. Such protracted periods of adaptive change, and the ability to trigger this response pharmacologically, are unprecedented findings. [unreadable] [unreadable] Studies proposed in this application are designed to begin to systematically elucidate the induction mechanisms whereby this long-lasting period of phenotypic adaptation is induced; we will focus on the involvement of the transcription factor 'hypoxia-inducible factor-1 alpha' (HIF-1a). Studies in the first aim will examine how HIF-1a protein expression is affected by our single and repetitive hypoxic and deferroxamine-based preconditioning regimens. Causal evidence for HIF-1a involvement in the preconditioning response will be forthcoming in Aim 2 studies utilizing HIF-1a knockout mice and oligonucleotide blockade of this transcription factor. The relationship of HIF-1a signaling to preconditioning-induced changes in nitric oxide production will be elucidated in Aim 3 studies, based on our observation that retinal ischemic tolerance cannot be achieved in nitric oxide synthase null mice. Understanding the molecular basis of these endogenous adaptive responses holds tremendous clinical promise, as these mechanisms can serve as new therapeutic targets for patients at risk for ischemic retinopathies and glaucoma. [unreadable] [unreadable] [unreadable]